This application claims the priority of German Patent Application Serial No. 100 14 910.3, filed Mar. 17, 2000, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a balancing hoist, and more particularly to a safety device for limiting the upper rotation speed of a balancing hoist.
Balancing hoists are known in many configurations. Examples include U.S. Pat. Nos. 5,553,832, 5,522,581, 5,556,077, and 5,439,200, The hoists described therein translate an axial movement of a hollow piston into a rotation of a cable drum or chain drum or chain sprocket. This conversion is implemented by a ball screw which carries a nut. Depending on the design, the ball screw may either be securely fixed to a housing while the nut is moved together with the cable drum along the spindle, or the nut is secured against axial displacement and allowed to rotate so that the ball screw is moved axially and prevented from rotating. Conventional hoists have a safety device in the form of several pawls which can rotate about a pivot axis in parallel relationship to the ball screw between a radially inward idle position and a radially outward brake position. Rotation of the pawls is realized in opposition to a spring force that tends to bias the pawls to seek the idle position. During rotation of the cable drum, the pawls point with their free end in rotation direction. As a consequence of the biasing restraining force of the spring, the pawls pivot into the brake position with their free end under the influence of centrifugal forces only when the cable drum reaches a predetermined rotation speed. In the brake position, the pawls are abruptly wedged into the stationary housing to thereby instantly stop the rotation of the drum or the chain sprocket with which the pawls are positively connected.
These conventional hoists suffer many shortcomings. As the pawls are driven abruptly into the housing, they are subject to impact stress as is also the attached assembly. The extent of this stress cannot be influenced. Thus, the related components may deform or break so that the attached assemblies and/or the hoist become useless. Therefore, once the safety device is triggered and damage is encountered, a performance test must be carried out and individual components must be replaced, if necessary. In these conventional balancing hoists, the performance test as well as the replacement of components of the safety device require a complicated disassembly during which the hoist must also be detached from the suspension.
It would therefore be desirable and advantageous to provide an improved hoist which obviates prior art shortcomings and to provide an improved a safety device which is simple in structure and allows simple replacement of components while yet being reliable in operation.
According to one aspect of the present invention, a balancing hoist includes a housing having a chamber accessible from outside, a piston movable by gas pressure in the housing in a longitudinal direction, a threaded spindle caused to rotate as the piston moves in the longitudinal direction, and carrying a nut which is securely fixed to the housing, a cable drum mounted in fixed rotative engagement to the spindle and movable longitudinally along the spindle, at least one pawl configured for rotation in opposition to an elastic restraining force between a radially inward idle position and a radially outward brake position about a pivot pin, which extends in parallel relationship to the spindle, with the pawl pointing in rotation direction at rotating cable drum and pivoting into the brake position as a result of centrifugal forces, when a rotation speed of the cable drum exceeds a predetermined level which is dependent on the magnitude of the restraining force, and at least one fixed stopper projecting radially inwardly from the housing at a level with the brake position for interaction with the pawl, when the pawl is in the brake position, for braking the rotation of the cable drum until its standstill, wherein the spindle has one end projecting into the chamber of the housing for connection to the pivot pin.
The present invention resolves prior art problems by providing a housing chamber which is accessible from outside and suitably closed by a cover, and in which the end of the threaded spindle projects for support of a braking mechanism which prevents the spindle from rotating beyond a predetermined rotation speed. This braking mechanism is in frictional engagement with the spindle so as to implement a gradual braking action of the spindle and thus of the cable drum. The stopper is stationary and projects inwardly from the housing wall into the travel path of the pawl, when the pawl is moved radially outwards into the brake position under the action of centrifugal force. Through arrangement of the braking mechanism in a separate housing chamber, which is easily accessible through detachment of the cover, operativeness can easily be checked and replacement of components, if necessary, is simple to carry out, without requiring a detachment of the hoist from a suspension.
According to another feature of the present invention, the stopper may be provided as an enlargement of the housing wall, with the enlargement projecting inwardly into the chamber.
According to another feature of the present invention, the frictional engagement of the braking mechanism with the spindle can be realized by fixedly securing an inner cone to the spindle, and by placing a complementary outer cone placed over the inner cone, whereby the pivot pin of the pawl is mounted to the outer cone. Thus, when the rotation of the cable drum exceeds the predetermined rotation speed, the centrifugal force acting on the pawl causes the pawl to impact upon the stopper to thereby gradually slow down the rotation of the outer cone and ultimately bring the spindle and the cable drum to rest, as a consequence of the frictional engagement between the inner cone and the outer cone. Suitably, the inner cone and the outer cone are braced together by a screw fastener